The present invention relates to a gas-feeding nozzle for use in an injection molding machine for the production of an injection molded article having an excellent appearance free of sink marks and distortion. More specifically, it relates to a gas-feeding nozzle for feeding a pressurized gas into a molten resin injected into a cavity of a mold attached to an injection molding machine for producing an injection molded article having a hollow structure.
For example, JP-A-64-14012 (corresponding to U.S. Pat. No. 4,740,150) discloses an injection molding machine for producing a molded article having an excellent appearance. JP-A-64-14012 discloses a method in which a molten thermoplastic resin is injected into a cavity of a mold, then a pressurized gas is introduced into the molten thermoplastic resin in the cavity to form a hollow structure within the thermoplastic resin. Finally, the gas within the hollow structure is released into the atmosphere before the mold is opened.
When the gas-feeding nozzle disclosed in JP-A-64-14012 is used, a gas ejection portion forming a top or forward portion of the gas-feeding nozzle is filled with part of the molten thermoplastic resin injected into the cavity before introduction of the pressurized gas into the molten thermoplastic resin. Then, the pressurized gas is introduced into the molten thermoplastic resin in the cavity through a gas ejection outlet at the top or forward end of the gas ejection portion. In this case, a gas flow path is formed in and along the central line of the molten resin filled in the gas ejection portion forming the top or forward portion of the gas-feeding nozzle as schematically shown in FIG. 17. During the introduction of the pressurized gas, the resin which is semi-molten or undergoing solidification is pressed onto the side inner wall of the gas ejection portion. As a result, the leakage of the pressurized gas through a gap between the gas ejection outlet and the mold can be prevented, and the pressurized gas can be reliably and smoothly introduced into the molten thermoplastic resin in the cavity of the mold. In FIGS. 17 and 18, numeral 100 indicates a conventional gas-feeding nozzle, numeral 112 indicates a gas ejection portion, numeral 120 indicates a gas ejection outlet, numeral 126 is a gas flow passage, numeral 130 is a non-return valve, numeral 42 indicates a movable mold member, numeral 44 indicates a gas inlet portion provided in the movable mold member 42, numeral 48 indicates a cavity, numeral 60 indicates a molten resin, numeral 62 indicates a hollow portion (structure) formed in the molten resin 60.
In the above prior art technique, however, when the pressurized gas is introduced through the gas-feeding nozzle into molten thermoplastic resin in the cavity, a phenomenon of the pressurized gas blowing off the resin filled in the gas ejection portion often takes place depending upon the injection molding conditions such as the type and viscosity of the resin used, pressure of the pressurized gas, introduction timing of the pressurized gas, and the like. This phenomenon will be hereinafter called "blow-off phenomenon", which is schematically shown in FIG. 18. When this phenomenon takes place, there arises the problem that the pressurized gas leaks, so that an injection molded article having an intended hollow structure can be no longer produced.
The blow-off phenomenon can be overcome in some cases by adjusting the pressure of the pressurized gas and the introduction timing of the pressurized gas, while it is often difficult to produce a molded article having an excellent appearance free of sink marks and distortion.